The most widely used on-site wastewater treatment systems for individual households have traditionally been either septic systems or aerobic treatment units. Septic systems generally include a septic tank followed by a leaching tile field or a similar absorption device located downstream, but physically on-site of the individual residence. The septic tank allows for larger/heavier solids in the sewage to settle out within the tank, while anaerobic bacteria partially degrade the organic material in the waste. The discharge from the septic tank is further treated by dispersion into the soil through any number of soil absorption devices, such as a leaching tile field, whereby bacteria in the soil continue the biodegradation process.
The conventional septic system is typically a flow-through system. The septic tank and the tile field are positioned so that sewage is carried out of the residence and through the treatment system by gravity and hydraulic displacement. As a flow-through system, the tank relies on sufficient hydraulic capacity to slow the velocity of the flow and allows settling of the solids to take place. Unfortunately, as the settable solids accumulate in the bottom of the tank, they displace the beneficial tank volume, effectively increasing the velocity of flow through the tank and decreasing the efficiency of solids removal. Also, as a flow-through system, the velocity of the flow through the tank and the related efficiency of solids removal by gravity are dependent upon the volume and frequency of the incoming sewage. A lower volume and rate of incoming sewage flow allows for greater gravity separation and removal efficiency. Higher volumes and rates of flow therefore decrease gravity settling and solids removal efficiency. Over the course of time, an increase in volume of organic material is discharged from the tank (due to decreasing removal efficiency) until the total volume of solids discharged over the life of the system exceeds the capacity of the downstream soil absorption system (leaching tile field) to accomplish further treatment. The soil absorption system will then retain solids and become plugged, thereby causing a back-up of sewage into the home. In this situation, the downstream soil absorption system is considered failed. Rejuvenation of a failed soil absorption system is not technologically feasible. Therefore, the downstream soil absorption system or other downstream device must be replaced or a new downstream device installed. However, even if sufficient land area is available toward the installation of a new downstream device, such can be accomplished only at considerable cost and inconvenience. Typically, heavy construction equipment is required to excavate and install any new replacement leaching tile field (a commonly used soil absorption system), or a similar device. This is much more inconvenient and costly than at the time of installation of the original treatment system. Construction equipment operating around an occupied residence frequently requires considerable destruction of hundreds of square feet of existing sod or lawn, moving fences, trees or recreational equipment, and creating a hazard for individuals, particularly smaller children.
Most aerobic treatment units are also flow through systems. Unlike septic tanks, aerobic treatment units perform primary (anaerobic) treatment and secondary (aerobic) treatment within the confines of the system. This arrangement provides a much higher degree of treatment within a relatively small area. As traditional aerobic treatment units are designed for a much higher removal of solids and organic compounds than anaerobic treatment units, a downstream device is frequently not required or is severely diminished in size compared to one which would be required downstream of a septic tank. In a traditional aerobic treatment unit, the first stage of the process is called pretreatment and provides for anaerobic treatment very much like that provided by a septic tank. A separate, isolated pretreatment chamber contains sufficient hydraulic capacity to slow the velocity of the flow somewhat and allows the settling of some of the solids to take place. Anaerobic bacteria partially degrade the organic material in the waste. As a flow through system, the contents of the pretreatment chamber (partially treated waste) are displaced by incoming sewage, and are transferred to the aeration chamber or biological reactor.
Within the aeration chamber, air is introduced in controlled amounts creating a proper environment for the development of a number of types of aerobic bacteria. The aerobic bacteria maintain a higher metabolic rate than anaerobic bacteria, which causes them to readily consume the organic material contained in the pretreated sewage. Prior to discharge of this flow through system, the aerobic bacteria (commonly called activated sludge) must be separated from the treated liquid. If the activated sludge particles are allowed to exit the system, two problems occur. First, the activated sludge would not be available to treat additional incoming sewage. As the system is operated on a continuing basis, the cultured bacteria need to be retained for future use. Secondly, if the activated sludge is allowed to be discharged from the system, the organic nature of the sludge would be considered a pollutant if returned directly to the environment.
Commonly, the activated sludge is separated from the treated liquid by allowing the solids to settle out in a gravity clarifier. In a flow through system, the contents of the aeration chamber containing the activated sludge are hydraulically displaced to the clarifier by partially treated liquid entering from the pretreatment chamber. Once in the gravity clarifier, quiescent conditions allow the activated sludge to slowly settle to the bottom of the chamber while the treated liquid is discharged from the system near the top of the chamber. The clarifier relies on having sufficient hydraulic capacity to slow the velocity of the flow through the chamber and thereby allows the activated sludge solids to settle to the bottom. The settled sludge at the bottom of the clarifier is returned, by various means, to the aeration chamber. This return prohibits the clarifier from accumulating a large volume of solids and thereby reducing the efficiency of solids separation. However, as a flow through system, the settling efficiency of the clarifier is dependent also on the volume and frequency of the incoming sewage flow.